As compared with conventional edge-emitting semiconductor lasers, VCSELs hold the promise of a number of desirable characteristics. For example, the shorter cavity resonator of the VCSEL provides for better longitudinal mode selectivity, and hence narrower linewidths. The use of multi-layered DBR mirrors to form a cavity resonator perpendicular to the layers obviates the need for the cleaving operation common to edge emitting lasers. This orientation of the resonator also facilitates both the fabrication of laser arrays and wafer-level testing of the individual lasers.
The prior art has proposed two basic VCSEL designs: one defines a current confinement region in a p-doped semiconductor DBR mirror by means of an apertured, high resistivity ion-implanted region (See, for example, Y. H. Lee et al.,Electr. Lett., Vol. 26, No. 11, pp. 710-711 (1990) and T. E. Sale, Vertical Cavity Surface Emitting Lasers, Research Press Ltd., pp. 117-127 (1995), both of which are incorporated herein by reference.), whereas the other defines the current confinement region by means of an apertured, high resistivity oxide layer (See, for example, D. L. Huffaker et al., Appl Phys. Lett., Vol. 65, No. 1, pp. 97-99 (1994) and K. D. Choquette et al., Electr. Lett., Vol. 30, No. 24, pp. 2043-2044 (1994), both of which are incorporate herein by reference.) In either case, the active region, frequently located in a mesa, is pumped by current which is made to flow through the aperture to the active region. In most VCSELs this pumping current also flows through the semiconductor layers of the DBR mirrors. In order to reduce free-carrier absorption, which can adversely affect laser efficiency, while also reducing the series resistance of the laser, the prior art has resorted to elaborate doping and compositional profile schemes. (See, for example, R. F. Nabiev et al., IEEE Photonics Technology Lett., Vol. 7, No. 7, pp. 733-735 (1995), which is incorporated herein by reference.) On the other hand, some VCSELs replace one or both semiconductor mirrors with dielectric mirrors. In this case a lateral injection structure is used to pump the active region. To date, the structure is simply a moderately doped semiconductor layer of uniform doping interposed between the current confinement aperture and one of the laser electrodes. (See, for example, M. H. MacDougal et al., IEEE Photonics Technology Lett., Vol 7, No. 3, pp. 310-312 (1996), which is incorporated herein by reference.) This layer serves to reduce free carrier absorption and to spread the current so that the full width of the VCSEL mesa is more uniformly pumped. However, the moderately doped layer of this design renders it difficult to make low resistance ohmic contact to the lateral injection layer. In addition, the diameter of the VCSEL is limited for practical doping concentrations because of the need for uniform current injection (i.e., uniform pumping of the active region).
Thus, a need remains in the art for a VCSEL design that allows for larger VCSEL mesas and thus higher power operation, while at the same time having relatively low series resistance.